Tires are typically prepared of treads of elastomer based rubber compositions which may contain particulate synthetic amorphous silica reinforcement.
Coupling agents are often used with the silica to assist in its reinforcement of elastomers. Such coupling agents typically contain a moiety (e.g. alkoxysilane based moiety) which is reactive with hydroxyl groups (e.g. silanol groups) contained on the surface of the silica and another, different, moiety (e.g. polysulfide based moiety) which is interactive with diene-based elastomers. Utilization of such silica coupling agents for such purpose is well known to those skilled in such art.
In practice, the rubber compositions are prepared by a sequential series of individual mixing steps. Where an internal rubber mixer is used, such mixing steps are conventionally composed of at least one non-productive mixing step in which the elastomer(s) and compounding ingredients are mixed to the exclusion of sulfur and sulfur vulcanizing accelerator followed by a productive mixing step in an internal rubber mixer in which the sulfur and vulcanization accelerators are mixed.
Conventionally, the materials are mixed in the non-productive mixing steps to a maximum mixing temperature in a range of, for example, from about 150° C. to about 180° C.
Conventionally, the materials, including the sulfur and vulcanization accelerators, are mixed in the productive mixing step at a maximum temperature, for example, in a range of about 90° C. to about 120° C. Between mixing steps, the mixture is conventionally dumped from the internal rubber mixer, perhaps then sheeted out into rubber sheets from a roll mill, and allowed to cool to 40° C. or lower prior to introducing the mixture to the next internal rubber mixing step.
For the practice of this invention, it has been observed that a significant benefit may be obtained in a nature of reduction in the viscosity of the rubber composition at the conclusion of the final non-productive mixing step as compared to the conclusion of the preliminary non-productive mixing step(s). Such reduction in Mooney viscosity was observed to be obtained in which, for preliminary non-productive mixing steps, the materials are mixed to a maximum mixing temperature in a range of about 150° C. to about 180° C. followed by a final non-productive mixing step to a reduced, or stepped down, maximum temperature in a range of about 90° C. to about 130° C. For the final non-productive mixing step in the internal rubber mixer, the rubber composition is simply re-mixed without any appreciable addition of ingredients. Sometimes such mixing without addition of other compounding ingredients is referred to as a “re-mill” mixing step even though the mixing itself is conducted in an internal rubber mixer. Such beneficial reduction in rubber composition mixing viscosity is considered herein to be significant in order to meet a Mooney viscosity target of the rubber mixture exiting the final productive mixing step in which the overall rubber mixing time in the internal rubber mixer mixing steps can be reduced. The target Mooney viscosity of the final rubber mixture is important for the subsequent processing of the compounded rubber mixture in various rubber processing equipment such as, for example rubber extruders for producing shaped unvulcanized rubber components for a tire such as, for example, treads and apexes as well as calendered cord reinforced plies. It is to be appreciated that a specific target Mooney viscosity will vary depending upon the rubber processing equipment being used and the rubber component being produced. Such desired Mooney viscosity phenomena for the processing of a compounded rubber composition is well known to those having skill in such art.
The term “non-productive” mixing, as hereinbefore discussed, is conventionally used to relate to one or more sequential mixing steps for mixing one or more diene-based elastomers with one or more ingredients without addition of free sulfur and sulfur vulcanization accelerator(s). The term “productive” mixing, as hereinbefore discussed, is conventionally used to relate to a mixing step following said non-productive mixing step(s) in which free sulfur and one or more vulcanization accelerator(s) are mixed with the rubber composition from the non-productive mixing step(s). Such sequential non-productive and productive mixing steps are well known to those having skill in such art. The term “carbon black” is used herein to refer to rubber reinforcing carbon blacks and such type of carbon black is well known to those having skill in such art. The term “silica-rich” refers herein to a rubber composition in which, insofar as its carbon black and silica reinforcement, particularly precipitated silica, fillers are concerned, the silica is in the majority of such fillers.
In the description of this invention, the term “phr” where used herein, and according to conventional practice, refers to “parts of a respective material per 100 parts by weight of rubber, or elastomer”.
In the description of this invention, the terms “rubber” and “elastomer” where used herein, are to be used interchangeably, unless otherwise prescribed. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art. The terms “vulcanize” and “cure” where used therein are used interchangeably unless otherwise indicated.